I've had the same experience
reports that the Seachem test is highly insensitive to EDTA or DTPA, requiring up to 24 hours to register properly with EDTA or 48 for DTPA (and showing about half the response even then.)
I've done some serious scrounging around online and it's surprisingly difficult to find information on how he Seachem test actually works. This makes it tough to know if there is anything you can do to speed up the test, such as adding something to break up the chelate or heating the test water, that won't interfere with the test. I'm going to continue scrounging.
So, I've dug a little more. Here is my best wild guess, which I'll emphasize up front could be completely wrong.
The reagent that keeps coming up in my internet searches is bathophenanthroline, which reacts with ferrous iron to produce an intense magenta color. That sounds like the Seachem test, all right. The stuff is pretty insoluble in neutral or alkaline water, but slightly soluble in acid water. I tested the pH of a little of the Seachem reagent in water, and, sure enough, it's quite acidic. My wild guess (emphasize again: my back-engineering of the Seachem test could easily be wrong) is that Seachem is a mixture of an acidifying agent, probably sodium acid sulphate (because it's cheap, nontoxic, and won't interfere with the test) with a small amount of bathophenanthroline. The other common and cheap nontoxic acidifying agent, citric acid, would interfere with the test.
Bathophenanthroline is a fairly expensive compound to manufacture, because it's a fairly complex organic compound. For those who are entertained by such things:
One site gives a spot price for the stuff of $178 for ten grams, but that may be pharmaceutical grade (it's used in medicine to test for iron in urine and blood serum), which will likely be more expensive than the technical grade good enough for a mass-produced aquarium test kit. Plus you only need a little in each container of test reagent -- the white powder you measure into the cuevettes is probably mostly the acidifying agent. The bathophenanthroline would still account for much of the cost of an iron test kit. The stuff is not notably toxic, so it's something Seachem can sell to aquarists without fear of liability.
Bathophenanthroline is a chelating agent, like citric acid, gluconate, EDTA, and DTPA, but with the property of turning intense magenta when it grabs onto a ferrous iron ion. So it's going to be competing with any chelating agent already in your aquarium water. I scrutinized the instructions that come with the Seachem kit; they claim their test works with "most" chelated iron but you have to be patient. That's the kind of careful wording you always have to watch out for. I suspect it does okay with citrated or gluconated iron, since those are relatively weak chelating agents, but it has a tough time competing with EDTA or DTPA for the ferrous iron.
Probably goes without saying, but it's almost certainly not worth the effort to try to order your own bathophenanthroline and acidifier to make your own iron test kit. Yours will work no better, you'll have to calibrate it, and you won't be able to negotiate anything better than the spot price for the reagents.
Any way to speed up the test? Probably not. Anything that can pry the iron away from the EDTA will likely also pry it away from the bathophenanthroline. EDTA is a pretty tough compound; living organisms can break it up to release the iron (else EDTA iron would be useless as fertilizer) but it's mostly destroyed in the environment by ultraviolet light from the sun. Which would also destroy bathophenanthroline.
reports that Hanna has a test that seems to do okay detecting EDTA-chelated iron. If this
is the test in question, then it uses phenanthroline, a simpler compound related to bathophenanthroline that turns orange rather than red. The test is (surprise!) a bit pricier, but if I continue to see a need to test iron, I may buy a kit and see how it works for me.